Both lightning and the electromagnetic pulse fields resulting from high-altitude nuclear bursts may couple electrical transients in the cables associated with power utility and communication systems. The electrical transients imparted to a cable or other conductor due to either of these sources may have adverse affects on any circuitry that may be connected thereto. Energy from such sources is referred to in the art as "high altitude electromagnetic pulses" (HEMPs). An electromagnetic wave of energy from the atmosphere can couple electrical energy into a power transmission cable. For HEMPs, the amount of energy that may be coupled into the cable is directly related to the length of the cable exposed to the plane wave; in other words, the longer the cable, the greater the coupled energy. This is a first order design parameter for determining the worst case threat condition that may have to be protected against. The susceptibility of a cable system to damage from HEMPs further depends on the type of cable, the energy of the waves, the physical dimensions of the cables' conducting and insulating layers, the electrical characteristics of the cables (including the breakdown voltage of the various insulation layers), and, when the cable is buried, the conductivity (or resistivity) of the ground and the depth at which the cable is buried.
Certain HEMP threats have been characterized by the electrical properties of their respective waveforms. The dominant characteristic of so-called "E1 threats" are their rapid voltage and current rise times ("time to peak"). The dominant characteristic of so-called "E2 threats" is the duration of their voltage and current peaks, i.e., the full-width half magnitude ("FWHM") of the waveform pulses. Another type of threat, the so-called "E3 threat," is characterized by differing initial and late waveforms. The primary concern in protecting a circuit from E3 threats is not the magnitude of the voltage pulse that may be coupled to the cable, but rather the energy, because such waveforms can melt metal. FIG. 5 herein tabulates approximations of the electrical properties of E1, E2, and E3 threats that are important in the art, the data having been obtained from Thevenin open and short circuit equivalents with the voltages being open circuit values that would exist at the terminals of a given cable when immersed in the electromagnetic field produced by a HEMP event and the currents being short circuit values of same. The significance of the data in FIG. 5 is in the order of magnitude of the electrical parameters. The units are specified alongside each entry in the Table, and generally consist of Amperes (A), microseconds (.mu.s), milliseconds (ms), nanoseconds (ns), Ampere-seconds (A-s), kilovolts (kV), and volt-seconds (V-s). FWHM values provide an indication of how long the pulse exists.
One type of cable for which HEMP protection is important is the deep water trunk (DWT) cable. DWTs connect underwater communications systems to ground-based circuitry located within a terminal equipment building (TEB). An electromagnetic barrier (EMB) within the TEB shields the protected circuitry from many external influences, including energy that may be coupled to the sheath of the DWT cable due to a HEMP. However, the core wire of the cable is electrically connected to a circuit which would be damaged if the HEMP were not attenuated. HEMP protection is required to attenuate such "residual current" to values which meet the specified requirements and which would otherwise pass through the EMB on the cable. In a HEMP protection context, the phrase "residual current" is the attenuated current in any HEMP protected conductor that passes through the EMB wall. Residual current has specified upper bounds in terms of di/dt and the maximum change in current following receipt of an E1 or E2 threat.
Conventionally, HEMP protection has been provided by using voltage limiting devices such as spark gaps, gas tubes, and lightning arrestors which, in effect, apply a short circuit. While such circuits effectively limit voltages to prescribed ranges, currents which are orders of magnitude greater than present during normal circuit operation result from this approach. These techniques are adequate in systems where voltage rather than current is the defined parameter, but not in systems in which current is the defined parameter. One example of a system in which current is the defined parameter is an undersea cable system. Abnormally large currents in such a system may damage, or even destroy, undersea cables as well as the main circuit which is to be protected.
What is needed in the art, and heretofore has not been available, is an ultrafast protection scheme that limits the current flowing toward a main circuit to a prescribed range.